1. Field of the Invention
The present invention relates to a solar simulator to measure current and voltage characteristics of a multi-junction photovoltaic device with high speed and precision, and a measuring method.
2. Description of the Background Art
An incident photon-to-current conversion efficiency of a photoelectric conversion element such as a photovoltaic device, a photovoltaic power element and an optical sensor is measured by measuring an electric current and voltage characteristics of the element under light irradiation. In case of characteristic measurement of a photovoltaic device, output characteristic curve is obtained by plotting collected data on the graph set up with voltage as horizontal axis and current as vertical axis. This curve is called an I-V curve generally.
As a measuring method, a method using sunlight as irradiating light or a method using an artificial light source as irradiating light is available. In the method using an artificial light source, a method using a fixed light source and a method using a flash source are known as patent documents 1 and 2.
By the method using a fixed light, tens of minutes are often necessary to make irradiance stable after putting the lighting on. The light source is needed to turn on continuously in order to keep the irradiance stable, but the temperature of the housing that holds the light source rise seriously. And since a part in the housing is always exposed to a light, various problems will occur, such of a degradation of an optical component as a mirror or an optical filter.
So, without using a fixed light, a method to measure an electric current and voltage characteristics of large area photovoltaic device by generating a flash has been proposed. A xenon lamp is used for a pseudo solar light source that generates a flash. And there are a single flash measuring method which uses one flash having a relatively long light-emitting time, and a short pulse flash measuring method which uses multiple flashes having a short light-emitting time.
However, by the single flash method, it is necessary to make long pulse over 100 msec to obtain I-V characteristic curve by sweeping a load of a photovoltaic device with a light emission. To emit such long pulse light, an interval between an emission and next emission needs to take a long time, so the measuring time will get longer. And, by emitting a long pulse light, a stress to the lamp will be large, so a lifetime of the lamp will be shortened.
By the method emitting multiple short pulse flash, a stress to the lamp is small because of the flashing, so emission can be performed with short interval. And as the emitting time is short, circumstances of an inside of the lamp such as temperature are hard to change, so peak irradiance tends to be stable. And because the pulse of the light which a photovoltaic device as a measurement object receives is short, the temperature of the measurement object will be hard to rise.
However, a wave shape of the short pulse flash is similar as mountain of which skirts are about 1 msec wide and the mountain don't has any flat area at a summit area. Thus, by once flashing, only one set of data as irradiance, a current and voltage can be collected. And, in measurement of slow response photovoltaic device, an output power may be measured as having low power because the output response cannot track fully the irradiance wave. 20 to 40 seconds are needed for a measurement, because the 60 to 120 times of flashing are needed.
Thus, in patent document 3, the following method is proposed to obtain an I-V curve. In the method, a flash having a flat top at the pulse wave is emitted toward a photovoltaic device by controlling a load thereto.
By the way, the spectrum of sunlight is extending from the ultraviolet rays to the infrared rays, and a single-junction photovoltaic device that has only one p-n junction cannot use lights over all wavelengths for power generation. Thus, a multi-junction photovoltaic device that connects plural p-n junctions in series is known. It generates electric power over wide wavelength with the multi-junction, so it can improve power generation efficiency.
In a multi-junction photovoltaic device, an upper layer or a top layer and a lower layer or a bottom layer are connected electrically in series. A top layer and a bottom layer have a different spectral sensitivity. A top layer has high sensitivity at short wavelength, and a bottom layer has high sensitivity at long wavelength. An electric charge of a bottom layer will increase in case that a spectrum shifts to long wavelengths, but an amount of power generation is restricted to the amount of power generation of a top layer because of the series connection. In this way, a multi-junction photovoltaic device has characteristics as power generation changes with a spectrum.
In the multi-junction photovoltaic device, an inspection using only one light source of xenon lamp as described in the patent documents 1, 2 and 3 has following problems. A spectrum of a xenon lamp has a lot of strong bright lines at long-wavelength side. The bright lines are attenuated by an optical filter, and used. But there are extremely strong bright lines near 820 nm and 900 nm, and those lines are difficult to be removed by an optical filter. In case that a bottom layer has spectral sensitivity near a range from 800 nm to 900 nm, output characteristics will change by the bright line. The characteristics of the optical filters has an unevenness which originates in production lots. Thus, an intensity of the bright line is different with respect to each inspecting apparatus. In case that a bottom layer of a multi-junction photovoltaic device has a spectral sensitivity at the aforementioned wavelength range, a measured value is different with respect to each inspecting apparatus.
In patent document 4, a solar simulator that has two light sources as a halogen lamp and a xenon lamp is proposed to solve the problem. Lights from a halogen lamp and a xenon lamp penetrate each optical filter and a xenon lamp will emit short wavelength light and a halogen lamp will emit long wavelength light, so an output characteristics of a multi-junction photovoltaic device is measured with a lowered effect from the bright line.
It is also available to change a balance of a long wavelength spectrum and a short wavelength spectrum by changing a power of the each light source. Thus, it is available to obtain power characteristics of which spectrum balance is adjusted finely.    Patent document 1: Japanese Patent No. 2886215    Patent document 2: Laid-open Japanese Patent Application No. 2003-31825    Patent document 3: Laid-open Japanese Patent Application No. 2007-88419    Patent document 4: Japanese Patent No. 3500352